1. Field of Invention
This invention relates to a process for producing improved clays for use in paper coating. More particularly, this invention relates to a process for the treatment of smectite clays for use in the coating of paper which enhances their theology, particle size distribution and abrasion characteristics.
2. Prior Art
Paper, which is generally a finely intermeshed mix of cellulose fibers, particularly cotton or linen fibers, is not well suited for printing purposes if left untreated because of transparency and irregularities on its surface. These deficiencies are generally corrected by the addition of binding agents, such as starch and resin, and by the incorporation into the fiber stock of various mineral fillers, such as calcium carbonate, calcium sulfate and especially clay. Thus, the quality of ordinary paper is greatly enhanced by coating its surface with a thin film of finely divided clay pigment particles and an adhesive mixture.
The clays used for paper coating are composed preferably of kaolinite or other similar clays. These clays must be produced into as pure as possible form for use in the coating of paper. In addition, it is important that the clays be easily dispersable in water and have exceedingly low viscosity. For best results the clays must yield a slurry of extremely high clay concentration with the lowest possible viscosity.
The use of water swellable smectite clays as a coating pigment for paper, wherein the coating weighs less than 5 g/m, is disclosed in European Patent Application No. 283 300. The preferred clay for use for this coating was a sodium smectite clay. The European application recommends that the natural bentonite be treated through an ion exchange process to convert the clay ions to predominately sodium ions. A method for processing kaolin clay by use of ion exchange resins to remove soluble impurities is disclosed in U.S. Pat. No. 4,451,440.
In the clay industry, crude clays are commonly washed to remove grit that may be present. In addition, raw clay is frequently allowed to stand to permit coarse particles to settle out. See, for example, U.S. Pat. Nos. 2,885,360 and 2,531,427. Additional treatments for clay including fractionation by settling or centrifugation are disclosed, for example, in U.S. Pat. Nos. 3,253,791, 4,018,673 and 4,451,440.
While various processes for the production of sodium smectite clays which are used in the paper coating industry have been disclosed, additional improvements in the processing of swellable clays are necessary to enhance the printing characteristics of the paper. To create these improved characteristics, the smectite-based clays must show improved rheological properties, particle size control and especially, low abrasiveness.
Unfortunately, commonly used sodium smectite clays exhibit high swelling volume, high viscosity and a high degree of thixotropy. These are undesirable properties for paper coating pigments. The variability of smectites with regard to rheology hinders the use of these products in the coating industry under normal operating conditions and has prevented their extensive use as coating pigments in general.
Accordingly, it is an object of this invention to produce improved smectite clay products for use in the coating of paper which exhibits reduced viscosity.
It is a further object of this invention to disclose an improved smectite clay for coating paper which minimizes the adverse thixotropy of natural and synthetic smectites, while still maintaining fine particle size and a high aspect ratio.
It is a still further object of this invention to disclose an improved smectite clay for use in the coating of paper which exhibits a reduction in the abrasiveness of the smectite clay.
It is a still further object of this invention to disclose a smectite clay for use in the coating of paper wherein the particle size of the clay product is carefully controlled.
These and other objects of the invention are obtained by the design of the process disclosed in the instant invention.